The technique of dual home/dual homing interconnection of L2 communication networks was developed for providing protection to the traffic carried between the networks. The most important requirement to the dual homing techniques is preventing traffic duplication and traffic loops which might occur due to intrinsic redundancy of the dual homing configurations. Various solutions of the technique are presently known in the prior art.
U.S.2008219156A describes a communication network protection system made up of interconnected networks, at least one of which has an automatic control plane (for example ASTM) comprising terminal interconnecting nodes, termed primary nodes. Corresponding secondary terminal nodes are associated with the primary terminal, wherein a secondary terminal node is used to realize a protection circuit in case of failure of the associated primary node.
U.S.2009168647A discloses interworking of an Ethernet Ring network with an Ethernet network with traffic engineered trunks (PBT network), that enables traffic engineered trunks to be dual homed to the Ethernet ring network to enable for protection switching between active and backup trunk paths in the PBT network. In one embodiment, the active path will terminate at a first bridge node on the Ethernet ring network and the backup path will terminate at a second bridge node on the Ethernet ring network. Trunk state information is exchanged between the bridge nodes to enable the bridge nodes to determine which of the active and backup paths should be used to forward data on the trunk. Upon a change in trunk state, a flush message is transmitted on the Ethernet ring network to enable the nodes on the Ethernet ring network to relearn the path to the new responsible bridge node.
U.S.2009168671A, incorporated hereby by reference, proposes yet another approach to the dual homing technology. To enable an Ethernet ring to be dual homed into a spanning tree protocol controlled Ethernet network, spanning tree control packets (Bridged Protocol Data Units or BPDUs) are transported as data frames over the Ethernet ring. This allows the Ethernet ring to appear as a single link to the spanning tree protocol so that the spanning tree can extend over the link. However, since the spanning tree does not have visibility as to the internal structure of the ring, the spanning tree cannot block links on the Ethernet ring network. Conversely, BPDUs from the Ethernet ring are not transmitted into the Ethernet domain that is implementing the spanning tree, so that the spanning tree is not affected by the control mechanism in place on the Ethernet ring network. As mentioned in the U.S.2009168671A, Ethernet nodes that are interconnected in a closed loop architecture, may be allowed to collectively run a separate control plane to control how data is passed between the nodes in the ring. The control plane on the closed loop selects one of the nodes to be a root node to provide for blocking of traffic flowing on the ring. This prevents traffic from endlessly looping on the ring. Additionally, the control plane provides for failure detection on the closed loop, notification of the failure to the nodes of the closed loop, and how connectivity can be restored to enable the closed loop to recover from failure. One aspect of the control protocol is that, upon failure in the closed loop, a fault indication message will be transmitted on the ring. Contribution 59 of Tejas Networks Ltd. to Study Group 15 of the ITU Telecommunication Standardization Section (COM15-C93-E), dated November 2008, proposes an extension to G.8032 to address the dual node interconnection between two rings (wherein G.8032 is a Ring networks protection protocol of the ITU-T standard committee). The proposal of the contribution is to denote links connecting the two rings as a so-called Interconnect Ring (IR) and circulating via the Interconnect Ring R-APS (control, signaling) messages of the G.8032 protocol. The proposal of the contribution requires differentiation of the R-APS messages in various ring networks and in the IR ring.
The important drawback of the above-mentioned proposal and of the other discussed prior art solutions as well is that all nodes of the dual homing structure (IR ring in the case of the above-mentioned contribution) must actively exchange G.8032 messages there-between. In other words, the dual homing nodes belonging to different networks must all be enabled to actively utilize the G.8032 protocol. Such a requirement seriously limits possibilities of a local network operator, who considers providing a dual homing connection with a remote network, since a loop protocol cannot be ensured by the local facilities.
Another drawback of conventional dual homing techniques stems from their intrinsic feature to provide protection to traffic carried between two interconnected networks. Traffic that for any reason does not require protection, and/or traffic assigned to pass between the networks via a specified connection of the dual homing configuration, is likely to be dropped in case of a physical cut or a logical blocking of this specified connection.
U.S.2008181196A discloses aggregating links across so-called multiple chassis connection (MC-LAG). When an indication is received that some local links are to be aggregated with some links on another chassis, coordination with the other chassis is performed, via an inter-chassis control path, to present (to a downstream equipment) the local links and the links on the other chassis as an aggregated group of links. The MC-LAG principle is currently (though not widely) used for pure L2 Ethernet networks only. However, it is not applicable to VPLS networks.